function wpse_77441_change_time_format( $anchor, $forum_id )
{
    $last_active = get_post_meta( $forum_id, \'_bbp_last_active_time\', true );

    if ( empty( $last_active ) ) {
        $reply_id = bbp_get_forum_last_reply_id( $forum_id );

        if ( !empty( $reply_id ) ) {
            $last_active = get_post_field( \'post_date\', $reply_id );
        } else {
            $topic_id = bbp_get_forum_last_topic_id( $forum_id );

            if ( !empty( $topic_id ) ) {
                $last_active = bbp_get_topic_last_active_time( $topic_id );
            }
        }
    }

      $date   = get_post_time( get_option( \'date_format\' ), $gmt, $reply_id, true );
      $time   = get_post_time( get_option( \'time_format\' ), $gmt, $reply_id, true );
      $dt = sprintf( _x( \'%1$s at %2$s\', \'date at time\', \'bbpress\' ), $date, $time );    

    $time_since = bbp_get_forum_last_active_time( $forum_id );


    return str_replace( "$time_since</a>", "$dt</a>", $anchor );
}
add_filter( \'bbp_get_forum_freshness_link\', \'wpse_77441_change_time_format\', 10, 2 );